Combustor

ABSTRACT

A combustor includes an end cap that extends radially across at least a portion of the combustor, wherein the end cap comprises an upstream surface axially separated from a downstream surface. A shroud circumferentially surrounds at least a portion of the end cap, wherein the shroud at least partially defines a fuel plenum between the upstream surface and the downstream surface. A combustion chamber downstream from the end cap defines a longitudinal axis. A plurality of tubes extend from the upstream surface through the downstream surface of the end cap to provide fluid communication through the end cap to the combustion chamber. A transition duct circumferentially surrounds at least a portion of the combustion chamber downstream from the end cap and curves tangentially from the longitudinal axis.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under Contract No. DE-FC26-05NT42643, awarded by the Department of Energy. The Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention generally involves a combustor. In particular embodiments of the present invention, the combustor may be incorporated into a gas turbine or other turbo- machine.

BACKGROUND OF THE INVENTION

Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles in the combustor where the compressed working fluid mixes with fuel and ignites in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases flow through a transition piece to the turbine where alternating stages of stationary nozzles and rotating buckets redirect, accelerate, and expand the combustion gases to generate work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.

Various design and operating parameters influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flame holding conditions in which the combustion flame migrates towards the fuel being supplied by the nozzles, possibly causing accelerated damage to the nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NO_(x)). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons. One solution for balancing the thermodynamic efficiency of the combustor, accelerated damage, and/or undesirable emissions over a wide range of combustor operating levels is to enhance mixing between the fuel and compressed working fluid to produce a lean fuel-working fluid mixture for combustion.

The enhanced mixing between the fuel and compressed working fluid is often accomplished by various combinations of injecting, atomizing, and/or swirling the fuel and/or working fluid prior to combustion to reduce localized hot spots in the combustion chamber. In some turbine designs, the stationary nozzles in the first stage of the turbine include rounded leading edges with large radii to accommodate swirling combustion gases impacting the first stage nozzles at various angles of incidence. In particular turbine designs, however, the first stage of stationary nozzles may be replaced with transition ducts between each combustor and the turbine. The transition ducts accelerate and redirect the combustion gases flowing into the turbine in place of the first stage nozzles. Although effective at enhancing turbine output and/or efficiency, excessive swirling in the combustion gases reduces the effectiveness of the transition ducts. As a result, an improved combustor design that enhances mixing between the fuel and working fluid without increasing swirling in the combustion gases would be useful to enhancing combustor performance without adversely affecting emissions.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention.

One embodiment of the present invention is a combustor that includes an end cap that extends radially across at least a portion of the combustor, wherein the end cap comprises an upstream surface axially separated from a downstream surface. A shroud circumferentially surrounds at least a portion of the end cap, wherein the shroud at least partially defines a fuel plenum between the upstream surface and the downstream surface. A combustion chamber downstream from the end cap defines a longitudinal axis. A plurality of tubes extend from the upstream surface through the downstream surface of the end cap to provide fluid communication through the end cap to the combustion chamber. A transition duct circumferentially surrounds at least a portion of the combustion chamber downstream from the end cap and curves tangentially from the longitudinal axis.

Another embodiment of the present invention is a combustor that includes an end cap that extends radially across at least a portion of the combustor, wherein the end cap comprises an upstream surface axially separated from a downstream surface. A fuel plenum is between the upstream and downstream surfaces, and a transition duct downstream from the end cap defines a longitudinal axis, a tangential axis, and a radial axis. A plurality of tubes extend from the upstream surface through the downstream surface of the end cap to provide fluid communication through the end cap to the transition duct. The transition duct includes an inlet and an outlet displaced from the inlet along the longitudinal axis and the tangential axis.

The present invention may also include a combustor having a fuel plenum and a combustion chamber downstream from the fuel plenum, wherein the combustion chamber defines a longitudinal axis. A plurality of tubes provide fluid communication from the fuel plenum to the combustion chamber, and a transition duct circumferentially surrounds at least a portion of the combustion chamber downstream from the plurality of tubes and curves tangentially from the longitudinal axis.

Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

FIG. 1 is a simplified side cross-section view of an exemplary gas turbine;

FIG. 2 is a simplified cross-section view of the exemplary combustor shown in FIG. 1 according to one embodiment of the present invention;

FIG. 3 is an enlarged cross-section view of a portion of the combustor shown in FIGS. 1 and 2 according to one embodiment of the present invention;

FIG. 4 is an enlarged cross-section view of a portion of the combustor shown in FIG. 1 according to an alternate embodiment of the present invention;

FIG. 5 is a partial perspective view of the end cap portion of the combustor shown in FIG. 4;

FIG. 6 is a downstream axial view of the end cap according to one embodiment of the present invention;

FIG. 7 is a downstream axial view of the end cap according to an alternate embodiment of the present invention;

FIG. 8 is a downstream axial view of the end cap according to an alternate embodiment of the present invention;

FIG. 9 is a perspective view of the transition duct and impingement sleeve shown in FIG. 2; and

FIG. 10 is a perspective view of multiple transition ducts circumferentially arranged around the gas turbine shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. In addition, the terms “upstream” and “downstream” refer to the relative location of components in a fluid pathway. For example, component A is upstream from component B if a fluid flows from component A to component B. Conversely, component B is downstream from component A if component B receives a fluid flow from component A.

Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Various embodiments of the present invention include a combustor that may be incorporated, for example, into a gas turbine or other turbo-machine. The combustor generally includes a plurality of premixer tubes that allow a fuel to be mixed with a compressed working fluid to produce a lean fuel-working fluid mixture with reduced amounts of swirl compared to conventional fuel nozzles. The lean fuel-working fluid mixture flows into a combustion chamber where it ignites to produce combustion gases having a high temperature and pressure. The combustion gases flow through a transition duct that accelerates and/or directs the combustion gases onto a first stage of rotating blades where the combustion gases expand and transfer energy to the rotating blades to produce work. Although exemplary embodiments of the present invention will be described generally in the context of a combustor incorporated into a gas turbine for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present invention may be applied to any combustor and are not limited to a gas turbine combustor unless specifically recited in the claims.

FIG. 1 provides a simplified cross-section view of an exemplary gas turbine 10 that may incorporate various embodiments of the present invention. As shown, the gas turbine 10 may generally include a compressor 12 at the front, one or more combustors 14 radially disposed around the middle, and a turbine 16 at the rear. The compressor 12 and the turbine 16 may share a common rotor 18 connected to a generator 20 to produce electricity.

The compressor 12 may be an axial flow compressor in which a working fluid 22, such as ambient air, enters the compressor 12 and passes through alternating stages of stationary vanes 24 and rotating blades 26. A compressor casing 28 contains the working fluid 22 as the stationary vanes 24 and rotating blades 26 accelerate and redirect the working fluid 22 to produce a continuous flow of compressed working fluid 22. The majority of the compressed working fluid 22 flows through a compressor discharge plenum 30 to the combustor 14.

The combustor 14 may be any type of combustor known in the art. For example, as shown in FIG. 1, a combustor casing 32 may circumferentially surround some or all of the combustor 14 to contain the compressed working fluid 22 flowing from the compressor 12. One or more fuel nozzles 34 may be radially arranged in an end cover 36 to supply fuel to a combustion chamber 38 downstream from the fuel nozzles 34. Possible fuels include, for example, one or more of blast furnace gas, coke oven gas, natural gas, vaporized liquefied natural gas (LNG), hydrogen, and propane. The compressed working fluid 22 may flow from the compressor discharge passage 30 along the outside of the combustion chamber 38 before reaching the end cover 36 and reversing direction to flow through the fuel nozzles 34 to mix with the fuel. The mixture of fuel and compressed working fluid 22 flows into the combustion chamber 38 where it ignites to generate combustion gases having a high temperature and pressure. A transition duct 40 circumferentially surrounds at least a portion of the combustion chamber 38, and the combustion gases flow through the transition duct 40 to the turbine 16.

The turbine 16 may include alternating stages of rotating buckets 42 and stationary vanes 44. As will be described in more detail, the transition duct 40 redirects and focuses the combustion gases onto the first stage of rotating buckets 42. As the combustion gases pass over the first stage of rotating buckets 42, the combustion gases expand, causing the rotating buckets 42 and rotor 18 to rotate. The combustion gases then flow to the next stage of stationary vanes 44 which redirect the combustion gases to the next stage of rotating buckets 42, and the process repeats for the following stages.

FIG. 2 shows a simplified cross-section view of the exemplary combustor 14 shown in FIG. 1 according to one embodiment of the present invention. As shown, the combustor casing 32 and end cover 36 may surround the combustor 14 to contain the working fluid 22 flowing from the compressor 12. An impingement sleeve 46 may surround the transition duct 40, and the working fluid 22 may pass through flow holes 48 in the impingement sleeve 46 to flow along the outside of the transition duct 40 to provide convective cooling to the transition duct 40. When the working fluid 22 reaches the end cover 36, the working fluid 22 reverses direction to flow through one or more fuel nozzles 34 and/or tubes 50 and into the combustion chamber 38.

FIG. 3 provides an enlarged cross-section view of a portion of the combustor 14 shown in FIGS. 1 and 2 according to one embodiment of the present invention. As shown, the one or more fuel nozzles 34 and tubes 50 may be radially arranged in an end cap 52 upstream from the combustion chamber 38. Various embodiments of the combustor 14 may include different numbers and arrangements of fuel nozzles 34 and tubes 50. For example, in the embodiment shown in FIGS. 1-3, the combustor 14 includes a single fuel nozzle 34 aligned with an axial centerline 54 of the combustor 14, and the tubes 50 are radially arranged around the single fuel nozzle 34 in the end cap 52. The fuel nozzle 34 may extend through the end cap 52 to provide fluid communication through the end cap 52 to the combustion chamber 38. The fuel nozzle 34 may include any suitable structure known to one of ordinary skill in the art for mixing fuel with the working fluid 22 prior to entry into the combustion chamber 38, and the present invention is not limited to any particular structure or design unless specifically recited in the claims. For example, as shown in FIG. 3, the fuel nozzle 34 may include a center body 56 and a bellmouth opening 58. The center body 56 provides fluid communication for fuel to flow from the end cover 36, through the center body 56, and into the combustion chamber 38. The bellmouth opening 58 surrounds at least a portion of the center body 56 to define an annular passage 60 between the center body 56 and the bellmouth opening 58. In this manner, the working fluid 22 may flow through the annular passage 60 to mix with the fuel from the center body 56 prior to reaching the combustion chamber 38. If desired, the fuel nozzle 34 may further include one or more swirler vanes 62 that extend radially between the center body 56 and the bellmouth opening 58 to impart swirl to the fuel-working fluid mixture prior to reaching the combustion chamber 38.

As shown in FIG. 3, the end cap 52 extends radially across at least a portion of the combustor 14 and generally includes an upstream surface 64 axially separated from a downstream surface 66. The tubes 50 generally extend axially from the upstream surface 64 through the downstream surface 66 of the end cap 52 to provide fluid communication for the working fluid 22 to flow through the end cap 52 and into the combustion chamber 38. Although shown as cylindrical tubes, the cross-section of the tubes 50 may be any geometric shape, and the present invention is not limited to any particular cross-section unless specifically recited in the claims. A shroud 68 circumferentially surrounds at least a portion of the end cap 52 to partially define a fuel plenum 70 between the upstream and downstream surfaces 64, 66.

A fuel conduit 72 may extend from the end cover 36 through the upstream surface 64 of the end cap 52 to provide fluid communication for fuel to flow from the end cover 36, through the fuel conduit 72, and into the fuel plenum 70. One or more of the tubes 50 may include a fuel port 74 that provides fluid communication from the fuel plenum 70 into one or more of the tubes 50. The fuel ports 74 may be angled radially, axially, and/or azimuthally to project and/or impart swirl to the fuel flowing through the fuel ports 74 and into the tubes 50. In this manner, the working fluid 22 may flow into the tubes 50, and fuel from the fuel conduit 72 may flow around the tubes 50 in the fuel plenum 70 to provide convective cooling to the tubes 50 before flowing through the fuel ports 74 and into the tubes 50 to mix with the working fluid 22. The fuel-working fluid mixture may then flow through the tubes 50 and into the combustion chamber 38.

FIG. 4 provides an enlarged cross-section view of a portion of the combustor 14 shown in FIG. 1 according to an alternate embodiment of the present invention, and FIG. 5 provides a partial perspective view of the end cap 52 portion of the combustor 14 shown in FIG. 4. As shown in FIGS. 4 and 5, the end cap 52 may again include the upstream surface 64, downstream surface 66, shroud 68, and fuel plenum 70 as previously described with respect to the embodiment shown in FIG. 3. In addition, the end cap 52 may include a generally horizontal barrier 74 that extends radially between the upstream surface 64 and the downstream surface 66 to axially separate the fuel plenum 70 from an air plenum 76. In this manner, the upstream surface 64, shroud 68, and barrier 74 enclose or define the fuel plenum 70 around the upstream portion of the tubes 50, and the downstream surface 66, shroud 68, and barrier 74 enclose or define the air plenum 76 around the downstream portion of the tubes 50. In particular embodiments, as shown most clearly in FIG. 5, one or more generally vertical baffles 78 may extend axially from the upstream surface 64 to the barrier 74 or completely through the end cap 52 to the downstream surface 66 to radially separate the tubes 50 into a plurality of groups or bundles 80 in the end cap 52. The baffles 78 (if present) allow each bundle 80 of tubes 50 to have a dedicated fuel plenum 70 and/or air plenum 76, allowing different fuels and/or fuel flow rates to be supplied to each bundle 80 of tubes 50. Alternately, the baffles 78 (if present) may include flow holes 82 or other perforations to facilitate the flow of fuel between the fuel plenums 70 associated with each bundle 80 of tubes 50.

As shown most clearly in FIGS. 4 and 5, the shroud 68 may include a plurality of air ports 84 that provide fluid communication for the working fluid 22 to flow through the shroud 68 and into the air plenum 76. In particular embodiments, as shown most clearly in FIG. 4, an air passage 86 between one or more tubes 50 and the downstream surface 66 may provide fluid communication from the air plenum 76, through the downstream surface 66, and into the combustion chamber 38. In this manner, a portion of the working fluid 22 may flow through the air ports 84 in the shroud 68 and into the air plenum 76 to provide convective cooling around the lower portion of the tubes 50 before flowing through the air passages 86 and into the combustion chamber 38.

Various embodiments of the combustor 14 may include different numbers and arrangements of fuel nozzles 34 and tubes 50, and FIGS. 6-8 provide downstream axial views of the end cap 52 illustrating various arrangements within the scope of the present invention. In the particular embodiment shown in FIG. 6, for example, the tubes 50 are radially arranged across the end cap 52, and fuel and working fluid 22 may be supplied through the tubes 50 to the combustion chamber 38. In the particular embodiment shown in FIG. 7, the generally vertical baffles 78 may separate the tubes 50 into generally circular tube bundles 80 radially arranged around a center circular tube bundle 80. Alternately, as shown in FIG. 8, the generally vertical baffles 78 may separate the tubes 50 into triangular or pie-shaped tube bundles 80 radially arranged around a center fuel nozzle 34. One of ordinary skill in the art will readily appreciate based on that teachings herein that the particular embodiments of the present invention are not limited to any particular arrangement, shape, or number of fuel nozzles 34, tubes 50, and/or tube bundles 80 unless specifically recited in the claims.

FIG. 9 provides a perspective view of the transition duct 40 and impingement sleeve 46 shown in FIG. 2, and FIG. 10 provides a perspective view of multiple transition ducts 40 radially circumferentially arranged around the gas turbine 10 shown in FIG. 1. As previously shown, the transition duct 40 generally surrounds at least a portion of the combustion chamber 38 and extends each end cap 52 and the turbine 16. In this manner, each transition duct 40 provides a path that conditions the flow of combustion gases from each combustor 14 to the turbine 16. In particular embodiments, the orientation and/or cross-section of the transition ducts 40 may replace or eliminate the need for stationary vanes 44 immediately upstream from the first stage of rotating buckets 42, thus increasing the efficiency and/or output of the turbine 16.

As shown in FIGS. 9 and 10, each transition duct 40 generally includes an inlet 90 and an outlet 92 downstream from the inlet 90. The cross-section of the inlet 90 generally conforms to the radial cross-section of the combustion chamber 38 proximate to the end cap 52, and the cross-section of the transition duct 40 may progressively narrow proximate to the outlet 92 to accelerate the combustion gases into the turbine 16. In addition, the transition duct 40 may curve between the inlet 90 and outlet 92 to enhance the angle at which the combustion gases flow into the turbine 16. For example, as shown in FIGS. 9 and 10, longitudinal, tangential, and radial axes 94, 96, 98 superimposed over the transition ducts 40 illustrate that the transition ducts 40 may curve transversely, tangentially, and/or radially from the longitudinal axis 94. It should be understood that the radial and tangential axes 96, 98 are defined individually for each transition duct 40 with respect to a circumference defined by the annular array of transition ducts 40, as shown in FIG. 10, and that the radial and tangential axes 96, 98 vary for each transition duct 40 about the circumference based on the number of transition ducts 40 disposed in the annular array about the longitudinal axis 94. As shown in FIGS. 9 and 10, the outlet 92 of the transition duct 40 may be displaced or offset from the inlet 90 along both the longitudinal and tangential axes 94, 98. In particular embodiments the transition ducts 40 may also curve radially from the longitudinal axis 94 to enhance the impact angle of the combustion gases against the rotating buckets 42. As a result, the outlet 92 of the transition duct 40 may be displaced or offset from the inlet 90 along the radial axis 96, as shown most clearly in FIG. 10. The combination of the tangential and/or radial offset of the outlet 92 with respect to the inlet 90 may obviate the need for stationary vanes 44 upstream from the first stage of rotating buckets 42.

The embodiments described and illustrated in FIGS. 2-10 provide one or more benefits over existing combustors and methods of supplying fuel to combustors. For example, conventional combustors often include fuel nozzles designed to swirl the fuel and working fluid to enhance mixing prior to combustion. Although effective at reducing undesirable NO_(x) emissions, a first stage of stationary vanes is often included between the combustor and the turbine upstream from the first stage of rotating buckets to redirect the resulting swirling combustion gases onto the first stage of rotating buckets. The transition duct incorporated into various embodiments of the present invention obviates the need for the first stage of stationary vanes, leading to enhanced efficiency of the turbine.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A combustor, comprising: a. an end cap that extends radially across at least a portion of the combustor, wherein the end cap comprises an upstream surface axially separated from a downstream surface; b. a shroud that circumferentially surrounds at least a portion of the end cap, wherein the shroud at least partially defines a fuel plenum between the upstream surface and the downstream surface; c. a combustion chamber downstream from the end cap, wherein the combustion chamber defines a longitudinal axis; d. a plurality of tubes that extend from the upstream surface through the downstream surface of the end cap, wherein the plurality of tubes provide fluid communication through the end cap to the combustion chamber; and e. a transition duct that circumferentially surrounds at least a portion of the combustion chamber downstream from the end cap, wherein the transition duct curves tangentially from the longitudinal axis.
 2. The combustor as in claim 1, further comprising a fuel port that provides fluid communication from the fuel plenum into one or more of the plurality of tubes.
 3. The combustor as in claim 1, further comprising an air plenum between the upstream and downstream surfaces and downstream from the fuel plenum.
 4. The combustor as in claim 3, further comprising one or more air ports that provide fluid communication through the shroud to the air plenum.
 5. The combustor as in claim 1, further comprising a fuel nozzle extending through the end cap, wherein the fuel nozzle provides fluid communication through the end cap to the combustion chamber.
 6. The combustor as in claim 5, wherein the plurality of tubes circumferentially surround the fuel nozzle.
 7. The combustor as in claim 1, further comprising a baffle extending axially from the upstream surface to the downstream surface, wherein the baffle separates the plurality of tubes into a plurality of tube bundles in the end cap.
 8. The combustor as in claim 1, wherein the transition duct curves radially from the longitudinal axis.
 9. A combustor, comprising: a. an end cap that extends radially across at least a portion of the combustor, wherein the end cap comprises an upstream surface axially separated from a downstream surface; b. a fuel plenum between the upstream and downstream surfaces; c. a transition duct downstream from the end cap, wherein the transition duct defines a longitudinal axis, a tangential axis, and a radial axis; d. a plurality of tubes that extend from the upstream surface through the downstream surface of the end cap, wherein the plurality of tubes provide fluid communication through the end cap to the transition duct; e. an inlet to the transition duct; and f. an outlet to the transition duct displaced from the inlet along the longitudinal axis and the tangential axis.
 10. The combustor as in claim 9, further comprising an air plenum between the upstream and downstream surfaces and downstream from the fuel plenum.
 11. The combustor as in claim 10, further comprising one or more air ports that provide fluid communication through the end cap to the air plenum.
 12. The combustor as in claim 10, further comprising an air passage between one or more of the plurality of tubes and the downstream surface of the end cap.
 13. The combustor as in claim 9, further comprising a fuel nozzle extending through the end cap, wherein the fuel nozzle provides fluid communication through the end cap to the transition duct.
 14. The combustor as in claim 9, further comprising a baffle extending axially from the upstream surface to the downstream surface, wherein the baffle separates the plurality of tubes into a plurality of tube bundles in the end cap.
 15. The combustor as in claim 9, wherein the outlet to the transition duct is displaced from the inlet along the radial axis.
 16. A combustor, comprising: a. a fuel plenum; b. a combustion chamber downstream from the fuel plenum, wherein the combustion chamber defines a longitudinal axis; c. a plurality of tubes that provide fluid communication from the fuel plenum to the combustion chamber; d. a transition duct that circumferentially surrounds at least a portion of the combustion chamber downstream from the plurality of tubes, wherein the transition duct curves tangentially from the longitudinal axis.
 17. The combustor as in claim 16, further comprising an air plenum between the fuel plenum and the combustion chamber.
 18. The combustor as in claim 16, wherein the transition duct curves radially from the longitudinal axis.
 19. The combustor as in claim 16, further comprising a baffle extending axially through the fuel plenum, wherein the baffle separates the plurality of tubes into a plurality of tube bundles.
 20. The combustor as in claim 16, further comprising a fuel nozzle in fluid communication with the combustion chamber, wherein the plurality of tubes circumferentially surround the fuel nozzle. 